<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <title>JS+HTML5酷炫星空背景动画特效</title>
        <style>
            * {
                padding: 0;
                margin: 0;
                overflow: hidden;
            }
            html,
            body {
                width: 100%;
                height: 100%;
            }
            #demo {
                width: 100%;
                height: 100%;
            }
        </style>
        <script src="jquery-3.1.1.min.js"></script>
    </head>
    <body>
        <div class="header" id="demo">
            <canvas id="canvas"></canvas>
        </div>

        <script>
            //宇宙特效
            'use strict';
            var canvas = document.getElementById('canvas'),
                ctx = canvas.getContext('2d'),
                w = (canvas.width = window.innerWidth),
                h = (canvas.height = window.innerHeight),
                hue = 217,
                stars = [],
                count = 0,
                maxStars = 2000; //星星数量

            var canvas2 = document.createElement('canvas'),
                ctx2 = canvas2.getContext('2d');
            canvas2.width = 100;
            canvas2.height = 100;
            var half = canvas2.width / 2,
                gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
            gradient2.addColorStop(0.025, '#CCC');
            gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
            gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
            gradient2.addColorStop(1, 'transparent');

            ctx2.fillStyle = gradient2;
            ctx2.beginPath();
            ctx2.arc(half, half, half, 0, Math.PI * 2);
            ctx2.fill();

            // End cache

            function random(min, max) {
                if (arguments.length < 2) {
                    max = min;
                    min = 0;
                }

                if (min > max) {
                    var hold = max;
                    max = min;
                    min = hold;
                }

                return Math.floor(Math.random() * (max - min + 1)) + min;
            }

            function maxOrbit(x, y) {
                var max = Math.max(x, y),
                    diameter = Math.round(Math.sqrt(max * max + max * max));
                return diameter / 2;
                //星星移动范围，值越大范围越小，
            }

            var Star = function () {
                this.orbitRadius = random(maxOrbit(w, h));
                this.radius = random(60, this.orbitRadius) / 8;
                //星星大小
                this.orbitX = w / 2;
                this.orbitY = h / 2;
                this.timePassed = random(0, maxStars);
                this.speed = random(this.orbitRadius) / 50000;
                //星星移动速度
                this.alpha = random(2, 10) / 10;

                count++;
                stars[count] = this;
            };

            Star.prototype.draw = function () {
                var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
                    y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
                    twinkle = random(10);

                if (twinkle === 1 && this.alpha > 0) {
                    this.alpha -= 0.05;
                } else if (twinkle === 2 && this.alpha < 1) {
                    this.alpha += 0.05;
                }

                ctx.globalAlpha = this.alpha;
                ctx.drawImage(
                    canvas2,
                    x - this.radius / 2,
                    y - this.radius / 2,
                    this.radius,
                    this.radius
                );
                this.timePassed += this.speed;
            };

            for (var i = 0; i < maxStars; i++) {
                new Star();
            }

            function animation() {
                ctx.globalCompositeOperation = 'source-over';
                ctx.globalAlpha = 0.5; //尾巴
                ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 2)';
                ctx.fillRect(0, 0, w, h);

                ctx.globalCompositeOperation = 'lighter';
                for (var i = 1, l = stars.length; i < l; i++) {
                    stars[i].draw();
                }

                window.requestAnimationFrame(animation);
            }

            animation();
        </script>
    </body>
</html>
